EP0729162B1 - Process for the fabrication of a material for electric contact composite - Google Patents

Description

The present invention relates to a method of manufacturing a material of composite electrical contact, particularly suitable for circuit breakers, this material being based on powder of a metal which is a good electrical conductor such as silver, copper or their compounds, and comprising carbon fibers distributed in the matrix metallic. It also relates to the material obtained by this process.

Such materials are commonly used to make pellets of contact in electromechanical devices such as circuit breakers or switches. The primary function of carbon or graphite present in the metallic matrix is to reduce the risk of contact soldering. However, the presence of graphite leads to increased mechanical and / or electrical erosion of the material.

When carbon is supplied as fibers - see for example the document US-4 699 763 - the resistance to erosion is certainly improved, but this improvement is acquired at the cost of a change in the behavior of the contact at the welding. The carbon fibers are mixed with the metal powder with contribution wetting agents, lubricants and solvents by the wet method, then drying is carried out, compression and sintering. The disadvantage of such a process is that it involves complications inherent in a wet process.

To find an acceptable compromise between the behavior of the material to erosion and its anti-welding behavior, it is proposed according to the document DE-41 11 683 to mix graphite particles with carbon fibers, this mixture being incorporated into the metal powder. By the hybrid contribution thus produced in the metallic matrix, one can obtain behaviors of the material to erosion and to the intermediate weld between those he would have shown with the addition of particles of graphite alone or addition of carbon fibers alone. But it turns out that under heavy loads and in particular under short-circuit current, particles of graphite, by their perfect crystal structure, show a tendency to be expelled from the surface of the material; this expulsion deteriorates the surface of the material in such a way that the carbon fibers are also expelled in turn. This results in an enrichment of the surface in silver and therefore an alteration of the qualities initially sought by the addition of graphite.

In document EP-171,339, a process for the manufacture of electrical contacts by impregnation of a carbon fiber substrate with a metal liquid under pressure, then by hot spinning of the mixture thus obtained. The fibers of carbon are severed during filament spinning oriented in the direction of spinning and having the drawback of having a length of between 15 μm and 150 µm, that is to say an average length significantly greater than 50 µm. We notes that the resulting contacts show too high a propensity to weld.

The object of the invention is therefore to avoid the drawbacks described and to allow a simple metalcarbon composite contact material to be produced with a fine, homogeneous and isotropic structure suitable for application in circuit breakers, this material showing good resistance to erosion both under nominal current and under short-circuit current, as well as a behavior satisfactory anti-welding and low and stable contact resistance.

• les fibres de carbone, dont la longueur initiale a une valeur moyenne prédéterminée, subissent un traitement mécanique d'attrition ou de broyage,
• les conditions de broyage sont déterminées de manière à engendrer des résidus de fibres de carbone broyées dont la longueur finale est répartie statistiquement de manière prédéterminée autour d'une valeur moyenne significativement inférieure à la valeur moyenne initiale,
• le mélange des résidus de fibres broyées à la poudre métallique est tel qu'ils sont distribués de manière isotrope dans le matériau.

According to the invention, in the method described in the preamble,

• carbon fibers, the initial length of which has a predetermined average value, undergo a mechanical attrition or grinding treatment,
• the grinding conditions are determined so as to generate milled carbon fiber residues whose final length is statistically distributed in a predetermined manner around an average value significantly lower than the initial average value,
• the mixture of the crushed fiber residues with the metal powder is such that they are isotropically distributed in the material.

In the material thus obtained, carbon is therefore exclusively present in the form of short carbon fiber residues, the statistical distribution of length is centered on the aforementioned final average value, which are distributed from homogeneously in the metallic matrix. The material shows good anti-welding behavior and low and stable contact resistance, at the same time time that, thanks to the semi-crystalline structure proper to all the elements carbonaceous materials inserted in the metallic matrix, low erosion under nominal current or under short circuit current. The compromise between these qualities is determined between others by the choice of the final average length of the fiber residues, preferably between 5 µm and 20 µm.

• La figure 1 est un diagramme illustrant les différentes étapes du procédé selon l'invention.
• La figure 2 en illustre une variante.
• La figure 3 montre la répartition statistique des longueurs de fibres de carbone avant et après broyage.
• La figure 4 est une micrographie de la surface du matériau après coupure sous courant de court-circuit.

The advantages and characteristics of the invention will emerge from the following description of exemplary embodiments, with reference to the appended drawings.

• FIG. 1 is a diagram illustrating the different stages of the method according to the invention.
• Figure 2 illustrates a variant.
• FIG. 3 shows the statistical distribution of the lengths of carbon fibers before and after grinding.
• FIG. 4 is a micrograph of the surface of the material after breaking under short-circuit current.

In the embodiment of the method which is illustrated in FIG. 1, we chooses commercially available carbon fibers of medium length L1 between approximately 100 µm and 800 µm, and delivered with a small standard deviation, and diameter between 4 and 20 µm. These fibers undergo mechanical treatment at cold and dry 10 in a ball mill, planetary mill or jet mill opposing air; the conditions of intensity and duration of the grinding make it possible to obtain fibers whose length is statistically distributed around an average value much lower than the initial mean value and with a standard deviation greater than the standard deviation initial, then are added in a weight proportion of approximately 2 to 5% by weight to the metallic powder. This powder is essentially a silver powder, or copper, or their alloys, of usual grain size, with possibly additives such as nickel, tungsten, titanium nitride, carbide tungsten or the like. Mixing the metal powder with crushed carbon fibers are carried out in a dry process in a mechanical mixer blades in step 20 until a homogeneous mixture is obtained, which then undergoes unit compression 30, then sintering 40, so as to obtain a structure of isotropic material.

In the embodiment illustrated in FIG. 2, the carbon fibers of the trade are mixed directly cold and dry with the metal powder in a mechanosynthesis step 21, for example in a ball mill or a planetary mill, such as the aforementioned grinding of the fibers takes place concomitant with mixing with metallic powder. The homogeneous mixture undergoes then, as before, unit compression 30 and sintering 40.

One thus obtains from fibers whose initial length L1 is 100 µm to 800 µm (see Figure 3) of the Gaussian distributions of fiber residues different lengths, according to a unimodal distribution of lower average value L2 at 20 µm.

Example : graphite fibers, with an initial average length of 300 µm, undergo attrition in a ball mill until residual fibers of an average length of less than 20 µm are obtained and are mixed in a weight proportion of 2 to 5% with silver powder. The weldability of the final material obtained is excellent and its erosion in electrical endurance is very low. FIG. 4 shows on a 280 scale a micrograph of the surface of the material after a 12 kA breaking test at 250 V; it can be seen that the contact surface retains after breaking under short-circuit current its fine, homogeneous and isotropic structure of fiber residues 50 of variable length and any orientation in the silver matrix 51.

By its short-circuit behavior, the material described is particularly suitable for use in circuit breakers.

Claims (5)

1. Method for manufacturing a composite electrical contact based on metal powder of a good electrical conductor such as silver or copper or a compound thereof, mixing carbon fibers with the powder, and then compressing the mixture and sintering it,
   characterized in that:

   the carbon fibers, whose initial length have a preset mean length (L1), undergo a mechanical grinding treatment in a grinder;

   the grinding conditions are determined such that they produce ground carbon fiber residues with a preset final length that is statistically distributed around a mean value (L2) that is significantly less than the initial value (L1);

   the ground fiber residues are mixed with the metal powder so that they are distributed isotropically in the material.
2. Method of claim 1 characterized in that the initial mean length (L1) of the carbon fibers is between 100 µm and 800 µm and the final mean length (L2) of the ground fiber residues is less than approximately 20 µm.
3. Method of claim 1 characterized in that the carbon fibers and the metal powder are mixed under dry conditions.
4. Method of claim 1 characterized in that the fibers are ground before they are mixed with the metal powder.
5. Method of claim 1 characterized in that the carbon fibers are ground in a manner that makes them suitable for being mechanically mixed with the metal powder.

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